Colon cancer develops through the stepwise accumulation of mutations. One of the earliest iransforming mutations occurs in the K-RAS proto-oncogene. To develop effective anticancer agents, it is mportant to elucidate effectors of oncogenic K-RAS. Two known effectors of oncogenic K-RAS signaling are MEK/ERK and cellular redox pathways. Recently, oncogenic K-RAS was found to be closely associated with the overexpression of glutathione S-transferase pi1 (GSTP1) in colon cancer. The biological significance of this association remains unclear. GSTP1, a member of the glutathione S-transferase enzyme superfamily, is widely overexpressed in colon cancer. GSTP1 has known antioxidant, detoxification, and stress signaling functions. As such, there has been considerable clinical interest in GSTP1 as a tumor marker and as a therapeutic target. The preliminary data in this proposal demonstrate that the presence or absence of oncogenic K- RAS determines cellular dependence on GSTP1. The mechanisms that underlie this observation are unknown. Recently, our lab reported that GSTP1 promotes MEK/ERK activation and protects against oxidative stress under growth-limiting conditions. These observations have led to the hypothesis that GSTP1 facilitates effective oncogenic K-RAS signaling by maintaining MEK/ERK activation and cellular redox. Three specific aims are proposed to test the above hypothesis: (1) To determine the mechanisms by which GSTP1 mediates oncogenic K-RAS activation of MEK and ERK, (2) To determine the mechanisms by which GSTP1 reduces oxidative stress generated by oncogenic K-RAS, and (3) To determine the role of GSTP1 in oncogenic K-RAS-promoted tumorigenicity in vivo. This proposal explores the paradox that while oncogenic K-RAS confers a mitogenic advantage to a cancer cell under growth-limiting conditions, it inadvertently renders the cell dependent on GSTP1. As such, a potential "Achilles Heel" may exist in colon cancers that harbor oncogenic K-RAS: they are more dependent on GSTP1 expression under growth-limiting conditions. This oncogenic dependence may eventually be exploitable for the therapy of colorectal cancer.